How can unwanted rotation of hair particles be avoided?
This sudden odd orientation change appears to be caused by the Armature modifier on the tree. The particles seem to respond much differently when the vertex positions are moving (it could possibly be because it is changing the size of the faces).

One way to combat this would be to move the perticle systems up the modifier stack until they are before the Armature modifier.

So instead of this:


You would instead end up with this:


It is important to note that when the armature moves the mesh, the particles might not move with it. You will need to convert to a mesh and add it to the armature deformation.